The present invention relates to speed-regulated sewing-machine drive systems of the type provided with a coupling, or coupling and braking, motor unit having an output drive member coupled in driving relationship to the main drive shaft of the sewing machine. An electromagnetically controlled friction coupling serves to couple the output drive member to the continuously rotating output shaft of the electric motor of the motor unit, and to decouple it therefrom, and also, if a coupling and braking motor unit is involved, to couple the output drive member to a brake and to decouple it therefrom. The electromagnetically controlled friction coupling is energized and deenergized by a negative-feedback regulator which compares a feedback voltage indicating the present rpm of the sewing machine against a command voltage indicating the manually or automatically selected rpm of the sewing machine, and in dependence upon the magnitude and/or polarity of the discrepancy between the two, causes the output drive member to become frictionally coupled to the constantly rotating drive shaft, or decoupled therefrom, or coupled to the brake of the unit, or decoupled therefrom. The feedback voltage is generated by a magnetic pole wheel driven in synchronism with rotation of the sewing-machine drive shaft, and is provided about its periphery with uniformly spaced circumferentially successive inductive elements, e.g., permanently magnetized portions, or permanent magnets, or projections or recesses in its unmagnetized ferromagnetic material, these successive inductive elements cooperating with inductive transducers distributed along the circumference of the pole wheel, with the voltage waveforms produced by each individual one of the plural inductive transducers being, in one way or another, combined together to form a voltage from which the feedback or actual-rpm voltage needed by the regulator can be derived.
An arrangement meeting the above description is known from Federal Republic of Germany published patent application DT-OS No. 2,134,751 in conjunction with U.S. Pat. No. 3,599,764. There, three inductive transducers are circumferentially spaced, one from the next, a distance of 120.degree. and their respective coils, in which in-phase voltages are induced, are connected together in series or in parallel. The thusly connected together inductive transducers produce at their combined output an A.C. voltage constituting the average value of the voltages furnished by the individual transducers individually, the amplitude of the A.C. output voltage being proportional to the rpm of the sewing machine, and its frequency being equal to the number of rotations or operating cycles performed by the sewing machine per second multiplied by the number of pole pairs of the magnetic pole wheel. Before this rpm-indicating voltage can be applied to a comparator or subtractor to generate an rpm-correction or actuating signal, this A.C. voltage must be rectified and then passed through a smoothing circuit. This requires additional circuit elements, and in particular the smoothing circuit requires space-consuming and in general undesirable smoothing capacitors.